Enabling Sentry Authorization for Impala

Authorization determines which users are allowed to access which resources, and what operations they are allowed to perform. In Impala 1.1 and higher, you use the Sentry open source
project for authorization. Sentry adds a fine-grained authorization framework for Hadoop. By default (when authorization is not enabled), Impala does all read and write operations with the privileges
of the impala user, which is suitable for a development/test environment but not for a secure production environment. When authorization is enabled, Impala uses the OS
user ID of the user who runs impala-shell or other client program, and associates various privileges with each user.

Note: Sentry is typically used in conjunction with Kerberos authentication, which defines which hosts are allowed to connect to each server. Using
the combination of Sentry and Kerberos prevents malicious users from being able to connect by creating a named account on an untrusted machine. See Enabling Kerberos Authentication for Impala for details about Kerberos authentication.

See the following sections for details about using the Impala authorization features:

The Sentry Privilege Model

Privileges can be granted on different objects in the schema. Any privilege that can be granted is associated with a level in the object hierarchy. If a privilege is granted on a
container object in the hierarchy, the child object automatically inherits it. This is the same privilege model as Hive and other database systems such as MySQL.

The object hierarchy for Impala covers Server, URI, Database, Table, and Column. (The Table privileges apply to views as well; anywhere you specify
a table name, you can specify a view name instead.) Column-level authorization is available in CDH 5.5 / Impala 2.3 and higher, as described in Column-level Authorization. Previously, you constructed views to query specific columns and assigned privileges based on the views
rather than the base tables.

A restricted set of privileges determines what you can do with each object:

SELECT privilege

Lets you read data from a table or view, for example with the SELECT statement, the INSERT...SELECT syntax, or
CREATE TABLE...LIKE. Also required to issue the DESCRIBE statement or the EXPLAIN statement for a query
against a particular table. Only objects for which a user has this privilege are shown in the output for SHOW DATABASES and SHOW TABLES
statements. The REFRESH statement and INVALIDATE METADATA statements only access metadata for tables for which the user has this
privilege.

INSERT privilege

Lets you write data to a table. Applies to the INSERT and LOAD DATA statements.

ALL privilege

Lets you create or modify the object. Required to run DDL statements such as CREATE TABLE, ALTER TABLE, or DROP TABLE for a table, CREATE DATABASE or DROP DATABASE for a database, or CREATE
VIEW, ALTER VIEW, or DROP VIEW for a view. Also required for the URI of the "location" parameter for the
CREATE EXTERNAL TABLE and LOAD DATA statements.

Privileges can be specified for a table or view before that object actually exists. If you do not have sufficient privilege to perform an operation, the error message does not disclose
if the object exists or not.

Originally, privileges were encoded in a policy file, stored in HDFS. This mode of operation is still an option, but the emphasis of privilege management is moving towards being
SQL-based. Although currently Impala does not have GRANT or REVOKE statements, Impala can make use of privileges assigned through
GRANT and REVOKE statements done through Hive. The mode of operation with GRANT and REVOKE statements instead of the policy file requires that a special Sentry service be enabled; this service stores, retrieves, and manipulates privilege information stored inside
the metastore database.

Starting the impalad Daemon with Sentry Authorization Enabled

To run the impalad daemon with authorization enabled, you add one or more options to the IMPALA_SERVER_ARGS
declaration in the /etc/default/impala configuration file:

The -server_name option turns on Sentry authorization for Impala. The authorization rules refer to a symbolic server name, and you specify the name to
use as the argument to the -server_name option.

If you specify just -server_name, Impala uses the Sentry service for authorization, relying on the results of GRANT and REVOKE statements issued through Hive. (This mode of operation is available in Impala 1.4.0 and higher.) Prior to Impala 1.4.0, or if you
want to continue storing privilege rules in the policy file, also specify the -authorization_policy_file option as in the following item.

Specifying the -authorization_policy_file option in addition to -server_name makes Impala read privilege information from
a policy file, rather than from the metastore database. The argument to the -authorization_policy_file option specifies the HDFS path to the policy file that defines
the privileges on different schema objects.

For example, you might adapt your /etc/default/impala configuration to contain lines like the following. To use the Sentry service rather
than the policy file:

The preceding examples set up a symbolic name of server1 to refer to the current instance of Impala. This symbolic name is used in the following ways:

In an environment managed by Cloudera Manager, the server name is specified through Impala (Service-Wide) > Category > Advanced > Sentry Service and Hive > Service-Wide > Advanced > Sentry Service. The values
must be the same for both, so that Impala and Hive can share the privilege rules. Restart the Impala and Hive services after setting or changing this value.

In an environment not managed by Cloudera Manager, you specify this value for the sentry.hive.server property in the sentry-site.xml configuration file for Hive, as well as in the -server_name option for impalad.

If the impalad daemon is not already running, start it as described in Starting Impala. If
it is already running, restart it with the command sudo /etc/init.d/impala-server restart. Run the appropriate commands on all the nodes where impalad normally runs.

If you use the mode of operation using the policy file, the rules in the [roles] section of the policy file refer to this same server1 name. For example, the following rule sets up a role report_generator that lets users with that role query any table in a database named
reporting_db on a node where the impalad daemon was started up with the -server_name=server1
option:

When impalad is started with one or both of the -server_name=server1 and -authorization_policy_file options, Impala authorization is enabled. If Impala detects any errors or inconsistencies in the authorization settings or the policy file, the daemon
refuses to start.

Using Impala with the Sentry Service (CDH 5.1 or higher only)

When you use the Sentry service rather than the policy file, you set up privileges through GRANT and REVOKE statement in
either Impala or Hive, then both components use those same privileges automatically. (Impala added the GRANT and REVOKE statements in
Impala 2.0.0 / CDH 5.2.0.)

Hive already had GRANT and REVOKE statements prior to CDH 5.1, but those statements were not production-ready. CDH 5.1 is
the first release where those statements use the Sentry framework and are considered GA level. If you used the Hive GRANT and REVOKE
statements prior to CDH 5.1, you must set up these privileges with the CDH 5.1 versions of GRANT and REVOKE to take advantage of Sentry
authorization.

For information about using the updated Hive GRANT and REVOKE statements, see Sentry service topic in the CDH 5 Security Guide.

Using Impala with the Sentry Policy File

The policy file is a file that you put in a designated location in HDFS, and is read during the startup of the impalad daemon when you specify both
the -server_name and -authorization_policy_file startup options. It controls which objects (databases, tables, and HDFS directory paths)
can be accessed by the user who connects to impalad, and what operations that user can perform on the objects.

The location of the policy file is listed in the auth-site.xml configuration file. To minimize overhead, the security information from this file is
cached by each impalad daemon and refreshed automatically, with a default interval of 5 minutes. After making a substantial change to security policies, restart
all Impala daemons to pick up the changes immediately.

Policy File Location and Format

The policy file uses the familiar .ini format, divided into the major sections [groups] and [roles]. There is also an optional [databases] section, which allows you to specify a specific policy file for a particular database, as explained in
Using Multiple Policy Files for Different Databases. Another optional section, [users], allows you to override the OS-level mapping of users to groups; that is an advanced technique primarily for testing and debugging, and is beyond the scope of this
document.

In the [groups] section, you define various categories of users and select which roles are associated with each category. The group and user names
correspond to Linux groups and users on the server where the impalad daemon runs.

The group and user names in the [groups] section correspond to Linux groups and users on the server where the impalad
daemon runs. When you access Impala through the impalad interpreter, for purposes of authorization, the user is the logged-in Linux user and the groups are the
Linux groups that user is a member of. When you access Impala through the ODBC or JDBC interfaces, the user and password specified through the connection string are used as login credentials for the
Linux server, and authorization is based on that user name and the associated Linux group membership.

In the [roles] section, you a set of roles. For each role, you specify precisely the set of privileges is available. That is, which objects users with
that role can access, and what operations they can perform on those objects. This is the lowest-level category of security information; the other sections in the policy file map the privileges to
higher-level divisions of groups and users. In the [groups] section, you specify which roles are associated with which groups. The group and user names correspond to
Linux groups and users on the server where the impalad daemon runs. The privileges are specified using patterns like:

For the server_name value, substitute the same symbolic name you specify with the impalad-server_name option. You can use * wildcard characters at each level of the privilege specification to allow access to all such objects. For example:

When authorization is enabled, Impala uses the policy file as a whitelist, representing every privilege available to any user on any object. That is, only
operations specified for the appropriate combination of object, role, group, and user are allowed; all other operations are not allowed. If a group or role is defined multiple times in the policy
file, the last definition takes precedence.

To understand the notion of whitelisting, set up a minimal policy file that does not provide any privileges for any object. When you connect to an Impala node where this policy file is
in effect, you get no results for SHOW DATABASES, and an error when you issue any SHOW TABLES, USE database_name, DESCRIBE table_name, SELECT, and or other statements
that expect to access databases or tables, even if the corresponding databases and tables exist.

The contents of the policy file are cached, to avoid a performance penalty for each query. The policy file is re-checked by each impalad node every 5
minutes. When you make a non-time-sensitive change such as adding new privileges or new users, you can let the change take effect automatically a few minutes later. If you remove or reduce
privileges, and want the change to take effect immediately, restart the impalad daemon on all nodes, again specifying the -server_name and -authorization_policy_file options so that the rules from the updated policy file are applied.

Examples of Policy File Rules for Security Scenarios

The following examples show rules that might go in the policy file to deal with various authorization-related scenarios. For illustration purposes, this section shows several very small
policy files with only a few rules each. In your environment, typically you would define many roles to cover all the scenarios involving your own databases, tables, and applications, and a smaller
number of groups, whose members are given the privileges from one or more roles.

A User with No Privileges

If a user has no privileges at all, that user cannot access any schema objects in the system. The error messages do not disclose the names or existence of objects that the user is not
authorized to read.

This is the experience you want a user to have if they somehow log into a system where they are not an authorized Impala user. In a real deployment with a filled-in policy file, a user
might have no privileges because they are not a member of any of the relevant groups mentioned in the policy file.

Examples of Privileges for Administrative Users

When an administrative user has broad access to tables or databases, the associated rules in the [roles] section typically use wildcards and/or
inheritance. For example, in the following sample policy file, db=* refers to all databases and db=*->table=* refers to all tables in
all databases.

Omitting the rightmost portion of a rule means that the privileges apply to all the objects that could be specified there. For example, in the following sample policy file, the
all_databases role has all privileges for all tables in all databases, while the one_database role has all privileges for all tables in
one specific database. The all_databases role does not grant privileges on URIs, so a group with that role could not issue a CREATE TABLE
statement with a LOCATION clause. The entire_server role has all privileges on both databases and URIs within the server.

A User with Privileges for Specific Databases and Tables

If a user has privileges for specific tables in specific databases, the user can access those things but nothing else. They can see the tables and their parent databases in the output of
SHOW TABLES and SHOW DATABASES, USE the appropriate databases, and perform the relevant actions
(SELECT and/or INSERT) based on the table privileges. To actually create a table requires the ALL
privilege at the database level, so you might define separate roles for the user that sets up a schema and other users or applications that perform day-to-day operations on the tables.

The following sample policy file shows some of the syntax that is appropriate as the policy file grows, such as the # comment syntax, \ continuation syntax, and comma separation for roles assigned to groups or privileges assigned to roles.

Privileges for Working with External Data Files

When data is being inserted through the LOAD DATA statement, or is referenced from an HDFS location outside the normal Impala database directories, the
user also needs appropriate permissions on the URIs corresponding to those HDFS locations.

In this sample policy file:

The external_table role lets us insert into and query the Impala table, external_table.sample.

The staging_dir role lets us specify the HDFS path /user/cloudera/external_data with the LOAD
DATA statement. Remember, when Impala queries or loads data files, it operates on all the files in that directory, not just a single file, so any Impala LOCATION
parameters refer to a directory rather than an individual file.

We included the IP address and port of the Hadoop name node in the HDFS URI of the staging_dir rule. We found those details in /etc/hadoop/conf/core-site.xml, under the fs.default.name element. That is what we use in any roles that specify URIs (that is, the locations of
directories in HDFS).

We start this example after the table external_table.sample is already created. In the policy file for the example, we have already taken away the
external_table_admin role from the cloudera group, and replaced it with the lesser-privileged external_table role.

We assign privileges to a subdirectory underneath /user/cloudera in HDFS, because such privileges also apply to any subdirectories underneath. If we
had assigned privileges to the parent directory /user/cloudera, it would be too likely to mess up other files by specifying a wrong location by mistake.

The cloudera under the [groups] section refers to the cloudera group. (In the demo VM used
for this example, there is a cloudera user that is a member of a cloudera group.)

Remember that to create a database requires full privilege on that database, while day-to-day operations on tables within that database can be performed with lower levels of privilege on
specific table. Thus, you might set up separate roles for each database or application: an administrative one that could create or drop the database, and a user-level one that can access only the
relevant tables.

For example, this policy file divides responsibilities between users in 3 different groups:

Members of the supergroup group have the training_sysadmin role and so can set up a database named training.

Members of the cloudera group have the instructor role and so can create, insert into, and query any tables in the
training database, but cannot create or drop the database itself.

Members of the visitor group have the student role and so can query those tables in the training database.

Using Multiple Policy Files for Different Databases

For an Impala cluster with many databases being accessed by many users and applications, it might be cumbersome to update the security policy file for each privilege change or each new
database, table, or view. You can allow security to be managed separately for individual databases, by setting up a separate policy file for each database:

Add the optional [databases] section to the main policy file.

Add entries in the [databases] section for each database that has its own policy file.

For each listed database, specify the HDFS path of the appropriate policy file.

To enable URIs in per-DB policy files, add the following string in the Cloudera Manager field Impala Service Environment Advanced Configuration Snippet (Safety
Valve):

JAVA_TOOL_OPTIONS="-Dsentry.allow.uri.db.policyfile=true"

Important: Enabling URIs in per-DB policy files introduces a security risk by allowing the owner of the db-level policy file to grant
himself/herself load privileges to anything the impala user has read permissions for in HDFS (including data in other databases controlled by different db-level policy
files).

Setting Up Schema Objects for a Secure Impala Deployment

Remember that in your role definitions, you specify privileges at the level of individual databases and tables, or all databases or all tables within a database. To simplify the
structure of these rules, plan ahead of time how to name your schema objects so that data with different authorization requirements is divided into separate databases.

If you are adding security on top of an existing Impala deployment, remember that you can rename tables or even move them between databases using the ALTER
TABLE statement. In Impala, creating new databases is a relatively inexpensive operation, basically just creating a new directory in HDFS.

You can also plan the security scheme and set up the policy file before the actual schema objects named in the policy file exist. Because the authorization capability is based on
whitelisting, a user can only create a new database or table if the required privilege is already in the policy file: either by listing the exact name of the object being created, or a * wildcard to match all the applicable objects within the appropriate container.

Privilege Model and Object Hierarchy

Privileges can be granted on different objects in the schema. Any privilege that can be granted is associated with a level in the object hierarchy. If a privilege is granted on a
container object in the hierarchy, the child object automatically inherits it. This is the same privilege model as Hive and other database systems such as MySQL.

The kinds of objects in the schema hierarchy are:

Server
URI
Database
Table

The server name is specified by the -server_name option when impalad starts. Specify the same name for all
impalad nodes in the cluster.

URIs represent the HDFS paths you specify as part of statements such as CREATE EXTERNAL TABLE and LOAD DATA. Typically, you
specify what look like UNIX paths, but these locations can also be prefixed with hdfs:// to make clear that they are really URIs. To set privileges for a URI, specify
the name of a directory, and the privilege applies to all the files in that directory and any directories underneath it.

In CDH 5.5 / Impala 2.3 and higher, you can specify privileges for individual columns, as described in Column-level Authorization. Formerly, to specify read privileges at this level, you created a view that queried specific columns
and/or partitions from a base table, and gave SELECT privilege on the view but not the underlying table.

URIs must start with either hdfs:// or file://. If a URI starts with anything else, it will cause an exception and the
policy file will be invalid. When defining URIs for HDFS, you must also specify the NameNode. For example:

Debugging Failed Sentry Authorization Requests

Sentry logs all facts that lead up to authorization decisions at the debug level. If you do not understand why Sentry is denying access, the best way to debug is to temporarily turn on
debug logging:

In Cloudera Manager, add log4j.logger.org.apache.sentry=DEBUG to the logging settings for your service through the corresponding Logging Safety Valve field for the Impala, Hive Server 2, or Solr Server services.

On systems not managed by Cloudera Manager, add log4j.logger.org.apache.sentry=DEBUG to the log4j.properties file on
each host in the cluster, in the appropriate configuration directory for each service.

which indicate each evaluation Sentry makes. The FilePermission is from the policy file, while RequestPermission is the privilege
required for the query. A RequestPermission will iterate over all appropriate FilePermission settings until a match is found. If no
matching privilege is found, Sentry returns false indicating "Access Denied" .

Managing Sentry for Impala through Cloudera Manager

To enable the Sentry service for Impala and Hive, set Hive/Impala > Service-Wide > Sentry Service parameter to the Sentry service. Then restart
Impala and Hive. Simply adding Sentry service as a dependency and restarting enables Impala and Hive to use the Sentry service.

To set the server name to use when granting server level privileges, set the Hive > Service-Wide > Advanced > Server Name for Sentry
Authorization parameter. When using Sentry with the Hive Metastore, you can specify the list of users that are allowed to bypass Sentry Authorization in Hive Metastore using Hive > Service-Wide > Security > Bypass Sentry Authorization Users. These are usually service users that already ensure all activity has been authorized.

Note: The Hive/Impala > Service-Wide > Policy File Based Sentry tab contains parameters only relevant to
configuring Sentry using policy files. In particular, make sure that Enable Sentry Authorization using Policy Files parameter is unchecked when using the Sentry
service. Cloudera Manager throws a validation error if you attempt to configure the Sentry service and policy file at the same time.

The DEFAULT Database in a Secure Deployment

Because of the extra emphasis on granular access controls in a secure deployment, you should move any important or sensitive information out of the DEFAULT
database into a named database whose privileges are specified in the policy file. Sometimes you might need to give privileges on the DEFAULT database for administrative
reasons; for example, as a place you can reliably specify with a USE statement when preparing to drop a database.

If this documentation includes code, including but not limited to, code examples, Cloudera makes this available to you under the terms of the Apache License, Version 2.0, including any required
notices. A copy of the Apache License Version 2.0 can be found here.